10.1 Final Conclusions and Summary
Element uptake with an emphasis on trace elements from the soil by different plants was investigated. The goal was to identify plants, plant species or plant cropping sys-tems with greater uptake of the elements Co, Ni, Mn and Mo in particular. This was a subproject of the joint research project "NiCo: Trace elements by energy crops – mass fluxes and recommendations for an optimized process biology in biogas plants" car-ried out by the GZG, Department of Sedimentology and Environmental Geology, Uni-versity of Göttingen, with contribution by the Faculty of Agronomy, Department of Crop Sciences in Göttingen, the DBFZ ("Deutsches Biomasseforschungszentrum") and the UFZ ("Umweltforschungszentrum"), the last two affiliations located in Leipzig, Germany.
The project was about treating trace element deficiencies in biogas plants, resulting from a high input rate of maize silage, with an addition of alternative energy crops. In many biogas plants in Germany (approximately 3000) trace element shortage is han-dled with addition of artificial trace element supplements. These supplements have a high concentration of trace metals, but the absolute concentration values are usu-ally unknown to the customer (biogas plant operator). This is problematic, as these supplements pose a health risk to human life. These metals are returned to the fields with the biogas residue and might enrich the soil with potentially toxic metals. Also, the needed trace element thresholds or the trace element concentrations in the fer-menter are also unknown to the biogas plant operators. The research project strives to provide better transparency to this aspect by publishing minimum threshold concen-trations of trace metals in biogas fermenters. This is part of a subproject performed by PhD-student Tino Pasold of the GZG.
In this thesis, the potential of alternative energy plants to supply trace elements to bio-gas plants in a natural way was investigated and the soil plant transfer characterized.
The aspects: soil parameters (pH, soil type, element concentrations, potentially avail-able elements), the influence of the plant species on element uptake, element contents in the plant tissue and the transfer of elements from soil to plant are covered.
The challenges of trace element determination, in particular for Co were investigated.
It was shown, that the majority of the plant samples (n=492, full mature plants) con-tained very low plant tissue concentrations of less than 0.1 mg Co/kg. It was stated, that Co and the trace element concentrations obtained by ICP-OES should be checked with a second analysis method (for examples ICP-MS), when available. The use of
112 Chapter 10. Final Remarks reference standard materials with Co values lower than 0.2 ppm (mg/kg) in plant samples is highly encouraged.
It was pointed out, that clean samples without dust/soil particles are important to characterize the plants’ uptake. It was shown, that plants grown in open field trials were vulnerable to adhering soil particles. From 1040 plant samples about 20 % (me-dian) of the Co measured was originating from soil particles (when a complete sample digestion process was performed). This was favored here for various reasons. To over-come this uncertainty, ratios of soil-borne elements in the soil should be checked in plant samples to detect soil particle influence, for example the ratio of Al to Ti. Three different methods were shown, to correct the element concentrations measured and to obtain concentrations close to the real plant uptake (physiological concentrations).
It was investigated, whether a higher element uptake results when plants were grown on a soil derived from basalt (close to the village of Bühren, Dransfeld, Lower Saxony).
The soil was rich in Fe, Mg, Co, Cr and Ni. Plants grown on a small scale field trial and plants grown in pots were investigated. For many plants elevated concentrations of Co, Ni, Mn, Fe and Zn were detected. The cereal crops (winter rye and winter triticale) showed no enrichment in Co and Ni on Bühren soil. It could be recommended to use large size pots for pot experiments or field trials, to allow the root system to develop freely.
A negative correlation between the concentration of trace metals Co, Ni, Mn and Fe in the soil extractions with ammonium nitrate and the soil pH was detected.
The most important outcome was that the plant species did have a distinctive element uptake, not only for the main nutrients, but also for a large range of elements. The different plant species did show different element concentrations and also diverse TF from soil to plant. The largest differences were often obtained for plant species be-longing to different plant families. Lowest trace element contents of Co and Ni were analyzed in Poaceae plants (maize, rye, triticale), and the greatest in Fabaceae plants (hairy vetch, summer and winter faba bean).
The pot and field experiments on soils with soil pH ranging from 4.6 to 7.3 and the soil extractions with ammonium nitrate showed an increased mobility of trace metals (Co, Ni, Mn, Cd and La) at low pH-values. This was an expected result, and this was assumed in the literature for these elements; but there is limited data published on Co and Ni content in plants. These elements are only favorable elements, but not essential in plant nutrition. Although the higher uptake at lower soil pH is an interesting sci-entific result, it cannot be used as a recommendation of action for farmers to increase their trace element uptake. As the soil pH is low, the DM yield will also be negatively affected.
Concluding with the results of the plant concentrations with the soil parameters, it can be stated that the soil pH is probably the most important factor governing uptake of Co, Ni, Mn and Fe. A high total concentration of these trace metals in the soil can attribute to a greater element concentration in the plants, as was observed for the plants grown on Bühren soil. The differences in uptake of the plant species was also mostly due to different plant families. On Bühren soil the Poaceae plants (triticale, rye) featured in most cases no elevated plant concentrations. In conclusion the factors concerning trace element uptake can be put in order according to their influence:
Soil pH (if < 6.5) < Total Content (Soil) < Plant Species.
10.2. Comments on Phytoremediation 113 The factorSoil pHonly takes effect at acidic conditions. The factorTotal Content (Soil) is determined by the geologic background. After these factors thePlant Speciesis most important. If the first two factors don’t change the greatest effect will have the plant species on trace element concentration in the plant.
A prediction of Co trace element concentration in the plant sample resulting only from soil analysis or soil extraction is difficult. For Co or Ni there was a linear trend towards greater plant tissue concentration with greater total soil, or greater concentration in the soil extractions with NH4NO3. This held true only for some plant species (winter faba bean or hairy vetch).
The concentrations in the plants were also combined with the DM yield to obtain total element removal from the fields and trace element delivery to biogas plants (Chapter 9). The plants which have the highest element concentrations, did also result in the largest element deliveries in the harvest material. That was because the DM yield of maize and for example faba bean differed by a factor of about two, whereas the Co and Ni concentrations of maize and faba bean differed by a factor of 10 and more.
This led to recommendations for farmers:
Mono grown summer faba bean or double croppings of winter faba bean (sole or in-tercropped with triticale) followed by maize did result in a good DM yield and in an increased trace element harvest, especially for Co and Ni. Ryegrass is a good crop to increase the amount of Mn, Mo and also Ni in the harvest.
With these recommendations additional environmental benefits will result, for exam-ple a greater biodiversity in the fields, without minimizing the farmers’ profit. It is unlikely that farmers take care of the trace element content in the harvest, while other parameters, like DM yield are much more (economically) important. One drawback is that faba bean can only be used in crop rotation every 5 years, because it is self-incompatible.
The trends of trace element delivery amounts were the same for the two main field sites with a high quality soil (Garte Nord from research farm Reinshof) and Sömmer-ling (near Uslar) with average quality soil. For common agricultural soils in western Europe without special geological backgrounds similar trends can be expected. It was pointed out, that with the calculated trace element concentration in the biogas fer-menter resulting from a hypothetical input mixture, it is unlikely that a pure plant mixture can deliver all trace element needs. A small addition of liquid manure is needed to guarantee sufficient supply of Co.
10.2 Comments on Phytoremediation
This study was about element uptake of a broad range of elements, including those with economic interest like Co, Ni or REEs. The working methods of researchers in the field of phytoremediation and in this study were similar. As a conclusion from all Chapters in this thesis (with emphasis on Chapter 4), important recommendations for the field of phytoremediation can be derived. For most of the metals of economic interest, it is important to make sure to perform a complete sample digestion, followed by a correction of adhering particles. In Chapter 4, it was pointed out, that the cutting height and also the plant height are crucial factors; small heights and low cutting heights can lead to a higher proportion of adhering soil particles. This soil proportion can alter the trace element concentrations and will feign high element uptake. When
114 Chapter 10. Final Remarks calculating metal extraction (phytoextraction) the plant yield should also be taken into consideration. Only then, it is possible to calculate the real time frame to "clean" the soil, which is rarely possible (Sauer and Ruppert, 2013; Sauer et al., 2017).
10.3 Critical Remarks and Outlook
In this study, the element data was corrected for adhering particles and dust, to cal-culate the element uptake of the different plant species. However, if the energy crops are ensiled and used as biogas substrate the adhering soil particles would contribute to a greater trace element supply. The biggest effect would probably have Co, which was corrected the most (from the trace elements essential for biogas production). For Mn, Mo and Ni this effect was negligible. Until now, research is lacking to identify whether these fine soil particles would be accessible to the microbes in the fermenter.
The challenging measurement of Co with the very small concentration levels in the upper plant harvest implies, that an overall reliable prediction model based on one easy applicable soil extraction method may not be realized. In order to implement a prediction method, a higher sample number, equal sample numbers per plant species, and more soil sample locations with increasing total or available element concentra-tions are needed to increase the reliability of the correlation.
One approach concerning the Fabaceae plants can pose the implementation of the N-fixation rate. The excretion of H+ -ions into the rhizosphere should increase with increasing N-fixation rate of the legume. To test this, a special experimental setting is needed, which could not be performed here. For example, with the application of N15 marked fertilizer, and a non-legume reference plant the N sources of the plants and the percentage of atmospheric N in the plant tissue can be calculated. Several plant samples with different N-fixation rates are needed to correlate the rate to the element content of Ni, Co, Mn or Fe.
However, this research was a valuable contribution to the field of plant nutrition over a broad element range, and to trace element supply for biogas production. From an ecological point of view, a higher diversity of plant substrates and less use of artificial trace element supplements for biogas plants will help to close element cycles. This avoids the risk of accumulating potentially toxic elements originating from the ele-ment suppleele-ments, when the biogas residue is returned to the fields. With the over-all results of other parts of the research project, for example the field parameters of Katharina Hey and the bioavailability and threshold experiments of Tino Pasold, a more complete picture can be drawn of this highly complex domain.
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